Method for adjusting the resonant frequency of an oscillating device

ABSTRACT

A method for increasing the resonant frequency of a torsional hinged device having a reduced attaching area between the torsional hinges and the supporting anchors. The resonant frequency is increased by adding a material over the reduced area to stiffen the connection between the torsional hinges and the support anchors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Patent Application of U.S. patentapplication Ser. No. 11/229,480 filed Sep. 16, 2005. This applicationrelates to the following co-pending and commonly assigned patentapplications: Ser. No. 11/228,893, Sep. 16, 2005, entitled ResonantOscillating Device Actuator Structure and Ser. No. 11/228,894, filedSep. 16, 2005, entitled Magnet On Frame Oscillating Device, whichapplications are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to the field of torsional hingedMEMS (Micro Electro Mechanical Systems) oscillating devices. Moreparticularly, the invention relates to a method of adjusting theresonant frequency of such an oscillating device having a powerefficient actuator structure for driving and maintaining oscillations ofthe torsional hinged device.

BACKGROUND

The use of rotating polygon scanning mirrors in laser printers toprovide a beam sweep or scan of the image of a modulated light sourceacross a photoresisted medium, such as a rotating drum, is well known.More recently, there have been efforts to use a much less expensive flatmember with a single reflective surface, such as a MEMS resonantoscillating mirror to provide the scanning beam. Other devices usingresonant oscillating members, other than mirrors, may also benefit fromthis invention. These torsional hinged resonant scanning devices provideexcellent performance at a very advantageous cost. However, every newtechnology has its own set of problems and resonant torsional hingeddevices, such as mirrors, are no exception.

As an example, inertially driven torsional hinged resonant devices madeof silicon exhibit unusually high mechanical gain. Further, suchresonant devices can readily be driven inertially through the support oranchor regions. However, as will be appreciated by those skilled in theart, the power required to drive the device to a required angularposition is a function of the stiffness of the torsional hinges at theanchor regions that support the device and stiffness of the anchorregion. Therefore, as customers demand larger mirrors or otheroscillating devices with higher and higher resonant frequencies, thetorsional hinges must be made stiffer. Typically, the greatest portionof the actuator or drive power is used to bend the anchor regions toprovide the rotational motion at the hinge attachment regions of theanchor. Since a favored source of power for an inertial drive is apiezoelectric element, it will be appreciated that as greater drivepower is required, greater drive voltages are also required to drive thepiezoelectric element. For battery powered applications, such highvoltage requirements are a problem.

Therefore, method and structures that facilitate the use of highfrequency large resonant devices with a large angular movement without acorresponding increase in drive power would be advantageous.

As will also be appreciated, the resonant frequency of such anoscillating device cannot be precisely set by manufacturing methods.Consequently, a method of adjusting the resonant frequency would be asignificant improvement and would improve yield.

SUMMARY OF THE INVENTION

This and other aspects and features are provided by a method foradjusting the resonant frequency of an oscillating device comprising thesteps of providing a resonant device comprising first and secondtorsional hinges for supporting an oscillating member, said first andsecond torsional hinges connected to first and second anchor membersrespectively, each of said first and second anchor members havingmounting portions and a central portion joined by a connecting region ofreduced stiffness and adding material over said defined connectingregion to increase the stiffness between said central portions and saidmounting portion of said anchor members to increase the resonantfrequency of said device.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIGS. 1A and 1B illustrate various types of prior art torsional hingedmirrors that can benefit from the teachings of the present invention;

FIGS. 2 is a perspective edge view and a side view of a torsional hingedmirror incorporating the teachings of the present invention;

FIGS. 3A, 3B, 4A, and 4B are top views and sectional side views of themirrors of the type shown in FIGS. 1A and 1B that have incorporated theteachings of the invention; and

FIG. 5 is a perspective view of a resonant torsional hinged mirror thatincludes material added to increase the stiffness between the torsionalhinges and support anchors according to the teachings of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

Referring now to FIGS. 1A and 1B, the two types of prior art torsionalhinged mirrors will be discussed. The selected and illustrated mirrorsare examples only and are in no way intended as limitations on the typesof torsional hinged mirrors may advantageously benefit from the presentinvention. FIG. 1A is a single axis single layer mirror device 10 andincludes a structure 12 that supports a frame member 14. An operatingsurface such as reflecting or mirror surface 16 is in turn supported bya single pair of torsional hinges 18 a and 18 b that lie along pivotingaxis 20. There is also shown a single permanent magnet 22 that could beused for monitoring the angular position of the operating surface ifsuch monitoring is necessary. However, it should be understood thatother techniques are available for monitoring the angular position ofthe mirror, and the permanent magnet may be eliminated. As indicated bydashed lines, anchor pads 14 a and 14 b may be used to support device 10rather than a support frame 14.

FIG. 1B is a multilayered single axis mirror. Elements of FIG. 1B commonto FIG. 1A and/or each other are labeled with the same referencenumbers. More specifically, as is shown in FIG. 1A, the device 10 a mayalso be supported by a frame member 14 or a pair of anchor members 14 aand 14 b.

The multilayered mirror of FIG. 1B is similar to FIG. 1A, except itincludes a hinge layer 16 a, a reflecting surface layer 16 b, and atruss layer 16 c for strengthening the reflecting surface and preventdeformation at resonant speeds. The hinge layer 16 c is unitary with thetorsional hinges 18 a and 18 b and is attached to frame 14 or anchors 14a and 14 b. The hinge layers are formed or etched from a single piece ofsilicon. There is also shown a permanent magnet 22 that may be includedfor monitoring purposes. The multilayered mirror is particularly usefulif a magnet 22 is used, as the thickness of the magnet and the trusslayer can be designed to assure the mass center of the structure lies onthe pivoting axis.

As will be appreciated by those skilled in the art, silicon is anexcellent spring material with very low losses. Consequently, the “Q” ofthe mirror structure made from silicon is very large (on the order of athousand or greater), and at resonance, there is a mechanicalamplification of approximately “Q” times the motion at the anchorregions. However, as the resonant frequency of a resonant scanningmirror is increased and the size (mass moment) of the mirror is alsoincreased, stiffer torsional hinges are required, which in turn requiresmore actuator or drive power. Resonant scanning mirrors that takeadvantage of the high mechanical gain at resonance may also beeffectively inertially driven through the anchor frame 14 or anchor pads14 a and/or 14 b. Piezoelectric inertia drive elements or actuators arean excellent inertia drive source. However, as mentioned above and/orwill be appreciated by those skilled in the art, the more drive powerrequired from a piezoelectric element, the higher the drive voltage thatis required. For many types of application, this is not a major problem.However, for some applications, and especially for battery drivenapplications, high voltage requirements may be a serious issue.

The above issue may be addressed by modifying the stiffness of theregions between the central portion and the mounting portions where thetorsional hinged device is anchored or attached to the inertial drivesource, (e.g. a piezoelectric element arrangement). The stiffness of theconnecting region is modified or reduced to provide more flexibilitybetween the mounting portions of the anchor members 14 a and 14 b orsupport frame 14 and the central portion of the anchor that is attachedto the torsional hinges 18 a and 18 b. This allows the up and downmotion of the piezoelectric elements to easily translate to rotation ofthe torsional hinges about their axis. More specifically, the stiffnessof the connecting region central portion of the anchor is decreased toprovide easier flexing of the actuators. This flexing generates theneeded angular rotation at the base of the torsional hinges. Moreimportantly this also means that the actuator requires less force todrive the device to a selected rotational angle.

FIG. 2 is a perspective edge view of a single layered mirror similar tothe prior art mirror of FIG. 1A, except it includes a reducedcross-sectional area, such as for example, a notched frame, fordecreasing the stiffness (or increasing the flexibility) according tothe present invention. As shown, the anchor portion 14 a and 14 b aswell as the torsional hinges 18 a and 18 b are etched from siliconhaving a selected thickness as indicated by arrow 28. As an exampleonly, if the thickness is 120 μm, then in the illustrated embodiment thenotches or trenches 30 a and 30 b that reduce the cross-sectional areaare etched to about 70% of the total thickness or approximately 85 μm.Also as shown in the embodiment of FIG. 2, the trenches or reduced areahave a width as indicated by arrow 32. In the example, the trenches ornotches have a width of about 125 μm so as to leave a central portion asindicated by arrow 34 (or as shown in the figure, about 250 μm). Itshould also be appreciated that the trenches that create the thinnedareas may be cut from the top surface or bottom surface of thestructure.

Referring now to FIGS. 3A, 3B, 4A, and 4B, there are shown a top viewand partial side cross sectional views of the present invention as couldbe incorporated in the mirror types of FIGS. 1A and 1B.

However, as will be appreciated by those skilled in the art, althoughcareful control of the manufacturing process in forming the torsionalhinged devices can provide devices, such as mirrors, that have aresonant frequency within a reasonable range. Each device of such agroup or batch of devices will likely have different resonantfrequencies, including some outside of an acceptable tolerance range.Therefore, if the resonant frequency of the device could be adjusted,the yield could be significantly increased, which would in turn lowerthe cost. The present invention provides a simple straight forwardtechnique for adjusting the resonant frequency of the device. As shownin FIG. 5, a quantity of material 36 a and 36 b is applied to the anchormembers 14 a and 14 b on the surface of the anchor member opposite thetrenches or areas of reduced thickness. The added material, such assolder or a UV curable adhesive, is of a type selected to stiffen theconnection or junction region where the central portion of the anchor isattached to the mounting portion. Stiffening the connection regionbetween the mounting portion and the central portion of the anchormember will result in an increase in the resonant frequency.Consequently, it will be appreciated that the design of the devicebefore adding the material should be designed with a bias toward a lowerfrequency.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine, methodsand steps described in the specification. As one of ordinary skill inthe art will readily appreciate from the disclosure of the presentinvention, machines, methods, or steps, presently existing or later tobe developed, that perform substantially the same function or achievesubstantially the same result as the corresponding embodiments describedherein may be utilized according to the present invention. Accordingly,the appended claims are intended to include within their scope suchmachines, means, methods, or steps.

1. A method for adjusting the resonant frequency of an oscillatingdevice comprising the steps of: providing a resonant device comprisingfirst and second torsional hinges for supporting an oscillating member,said first and second torsional hinges connected to first and secondanchor members respectively, each of said first and second anchormembers having mounting portions and a central portion joined by aconnecting region of reduced stiffness; and adding material over saiddefined connecting region to increase the stiffness between said centralportions and said mounting portion of said anchor members to increasethe resonant frequency of said device.
 2. The method of claim 1 whereinthe stiffness of said connecting region is reduced by decreasing thecross-sectional area of said connecting region.
 3. The method of claim 2wherein said step of decreasing the cross-sectional area comprises thestep of thinning the material of said connecting region.